The present invention relates to the preparation of ethylene dichloride by the addition chlorination of ethylene. In its preferred form, the invention relates to an improved process for preparing 1,2-dichloroethane by addition chlorination, wherein heat generated by the exothermic reaction of chlorine and ethylene is used to vaporize and rectify the ethylene dichloride produced.
The preparation of dichloroalkanes by the chlorination of an appropriate olefin in the liquid phase under suitable conditions is well known. U.S. Pat. No. 2,929,852, British Pat. No. 1,231,127, British Pat. No. 760,308, and DT No. 2224253 disclose methods of preparing dichloroalkanes wherein an olefin is addition chlorinated (with chlorine) in the liquid phase at suitable temperatures. According to these patents, the dichloroalkane formed is distilled or vaporized, and is passed to further treatment. In the case of U.S. Pat. No. 2,929,852 and British Pat. No. 1,231,127, the vaporized dichloroalkane is passed to a closely related fractional distillation column where the heat of reaction of chlorine and ethylene is used to fractionate the vaporized dichloroalkane. British Pat. No. 1,231,127 also provides for the fractionation of dichloroalkane obtained from at least one other source. According to this patent, the heat of reaction contained in the vaporized dichloroalkane is sufficient to rectify the dichloroalkane vapor, and because of the large excess of heat present, may be used additionally to rectify crude dichloroalkane from another source, e.g., crude dichloroalkane obtained from the oxychlorination of an olefin, and/or unconverted recycle dichloroalkane from a pyrolysis system in which dichloroalkane is pyrolyzed to a given monochloroalkene.
The procedures outlined in these patents suffer from a number of disadvantages. For example, a number, if not all, of these procedures employ a boiling liquid medium for carrying out the chlorination reaction. One difficulty with such a system is that if vaporization occurs at the site of reaction, the vapor that is formed acts as a stripping gas to strip unreacted chlorine and ethylene before they can react or dissolve. Poor conversions and selectivity may result.
An additional problem associated with at least one of the prior art processes relates to bubble formation. In this prior art process, ethylene and chlorine are introduced at a slow rate into the bottom of a tank reactor which is partially filled with ethylene dichloride and catalyst. The ethylene dichloride formed is vaporized from the tank and conducted to a fractionation column where it is purified. However, if attempts are made to increase the rate of addition of ethylene and chlorine, all of the ethylene does not dissolve, and tends to form gas bubbles which pass, unreacted or partially-reacted, up the liquid and out of the tank. Thus, this prior art process is limited in terms of throughput, and must be maintained at relatively slow rates of addition of reactants.
Moreover, if feed rates are increased, and more chlorine and ethylene do react, the increased heat given off promotes increased boiling, and may result in a safety problem as well as in the formation of by-products. In another prior art process, where increased rates of addition of ethylene and chlorine are maintained, an external heat exchange system is required in order to regulate bubble size.